Additional analyses from the SOLAR-1 study show that alpelisib (BLY719), an investigational alpha-specific PI3K inhibitor, combined with fulvestrant (Faslodex) extended progression-free survival (PFS) compared with fulvestrant alone in patients with PIK3CA-mutant advanced breast cancer regardless of line of therapy or prior CDK4/6 inhibitor treatment. An interim overall survival (OS) analysis also demonstrated that the median OS has not been reached in the alpelisib arm.1

The study also demonstrated the potential clinical utility of measuring circulating tumor (ct) DNA to select patients with a PIK3CA mutation and to confirm the robust PFS benefit with alpelisib in patients with plasma ct-DNA-determined mutational status, said Dejan Juric, MD, at the 2018 San Antonio Breast Cancer Symposium.

Approximately 40% of estrogen receptor (ER)-positive breast cancers harbor mutations in the PIK3CA gene, encoding the PI3 kinase alpha form of PI3 kinase.

“This alteration is the most common actionable alteration in ER-positive breast cancer,” said Juric, director, Henri and Belinda Termeer Center for Targeted Therapies, Massachusetts General Hospital, Boston. “PI3 kinase signaling itself promotes ER-independent growth of ER-positive breast cancer cells, and it’s implicated in resistance to antiestrogens, which is reversed by combined PI3 kinase and ER inhibition.”

There are 4 PI3K isoforms. Alpelisib inhibits just the alpha isoform, which potentially results in a much wider therapeutic index compared with pan-PI3K and beta-sparing CDK4/6 inhibitors, which target multiple isoforms.

Among updated data presented here, the first interim OS analysis after 52% of the planned number of events demonstrated a positive OS trend to the combination in the PIK3CA-mutant cohort, he said, with an HR of 0.73 (95% CI, 0.48-1.10; P = .06). Two additional OS analyses “will shed a lot more light on the potential OS benefit of this combination,” Juric said.

In the PIK3CA-mutant cohort, target lesion diameter decreased by 75.86% from baseline in the alpelisib plus fulvestrant arm, compared with a 43.51% decrease in the fulvestrant-only arm.

Analysis of PFS by prior lines of therapy “reveals increasing benefit of alpelisib as we go from endocrine-sensitive first-line patients, a small portion of patients in this study, to endocrine-resistant first-line patients to second-line patients,” he said.

In addition to the tissue-based mutation profiling used for the primary analysis of SOLAR-1, plasma ctDNA samples were collected at baseline and analyzed by polymerase chain reaction to retrospectively assess PFS by PIK3CA mutation status as a secondary endpoint. Based on this analysis, combined treatment with alpelisib and fulvestrant resulted in a median PFS of 10.9 months compared with 3.7 months with fulvestrant alone (HR, 0.55) in those with PIK3CA mutation. In the non-mutant cohort, median PFS was 8.8 and 7.3 month, respectively (HR, 0.80).

“This analysis clearly illustrates the potential clinical value of this easily accessible biospecimen, allowing for a rapid determination of PIK3CA mutational status right at the time of initiation of therapy, as opposed to relying on often distant archival tumor material,” he said.